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Subtask C: Improved Techniques for Solar Resource
Characterization and Forecasting
Following the 2nd Experts Meeting
in Denver, the Subtask was given a new clearer structure. Contents mainly remain
constant, but the related work is now grouped in only 3 instead of 7 activities.
Reporting of 2006 Subtask C achievements follow the new Subtask C structure
below.
C.1 Improving Satellite Methods for Solar
Radiation Products:
This now includes the ‘solar micro-siting’
action, which is the improvement of mainly satellite-based methods towards the
1 km scale. Optionally this activity can also cover the development of spectral
and angular-resolved solar products derived from satellite data. This activity
will be led by Prof. Richard Perez, SUNY, USA.
C.2 Climatological Analysis of Solar Resources
This activity analyzes long-term changes in
solar radiation conditions based on measurements, satellite data and climate and
weather models (see Figure 7.1 for an example of the output of this task). If
solar energy becomes more important and funding becomes available, actions for
seasonal forecasting and decadal prediction would be added to this activity.
This activity is led by Dr. Paul W. Stackhouse, NASA, USA.
C.3 Forecasting of Solar Radiation
This activity now covers all forecast horizons
from now-casting up to a few hours to short-term forecasting up to 3 days and
mid-term forecasting up to 2 weeks. Most emphasis at the moment is on
now-casting and short-term forecasting. The mid-term horizon could be added if
funds become available. This activity is led by Dr. Elke Lorenz, Universität
Oldenburg, Germany.
C.1: Improving Satellite Methods for Solar Radiation Products
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NASA/LaRC and SUNY/A evaluated the use of sub-grid
variability to increase resolution of long-term data products) (Perez et
al., Proc. of the ASES Solar 2006 conference, Denver, July 2006).
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Work on a concept paper for solar micro-siting started during
a visit of a NASA/LaRC scientist to DLR IPA in Oberpfaffenhofen. The paper
discusses options for a field campaign, which should provide new insights
into the nature of the solar radiation field at highest temporal and spatial
resolution. The basic plan is to spread many radiometers around a central
station with a roughly logarithmical increase in spacing. Intervals should
start at around 100 m and increase to several kilometers to bridge the gap
from very high resolution in the center up to large boxes as covered by the
NASA-ISCCP, DLR-ISIS or NASA-SSE datasets. This work will contribute to the
development and validation of new retrievals employing operational satellite
data with a spatial resolution in the 1 km range, and allow for better
understanding of the scaling effects and representativeness of single sites
for large boxes, which also improves validation options for the medium to
large-scale solar maps.
C.2: Climatological Analysis of Solar Resources
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NASA/LaRC prepared for the reprocessing of SSE Release 6.0.
This shall result in improved solar irradiance maps based on more than 20
years of data. This is an action, which is also part of the Radiative Flux
Assessment (RFA) of GEWEX (Global Energy and Water Cycle Experiment), which
also covers analysis of satellite datasets vs. long-term surface
measurements sites, and inter-comparison with other products like DLR-ISIS
(presented at ASES).
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NASA/LaRC manages databases and results of the GEWEX
Radiative Flux Assessment.
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The DLR-ISIS (Irradiance at the surface based on ISCCP)
dataset was completed and now covers the full time period from July 1983 to
end of 2004 in 3 hourly time-steps. As it is based as closely as possible on
real atmospheric optical depth and covers more than two decades, it makes it
possible to analyze changes in irradiance conditions during this long-time
period. The DLR-ISIS data set is available at the website
http://www.pa.op.dlr.de/ISIS.
This global dataset offers global horizontal irradiance as well as direct
normal irradiance (see Figure 7.1).
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The Ph.D. thesis of Sina Lohmann and a related paper in the
Journal of Solar Energy by Lohmann, et al. went to press. They describe the
DLR-ISIS method and first results with inter-comparison to reanalyze data.
Results on solar irradiance derived from atmospheric models do not seem to
be able to reproduce observations. This reduces chances that climate models
and seasonal forecasting of solar irradiance will be successful soon.
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Collaboration of DLR-IPA with the University of Oregon in
Eugene led to two papers describing long-term changes in solar irradiance in
the Pacific Northwest USA. Another paper of Lohmann et al. (accepted for
publication in Geophysical Research Letters) shows an increase in
availability of direct irradiance since the late 1970s, while another paper
by Riihimaki et al. at the ASES conference reveals relatively stable
conditions for global horizontal irradiance.
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The inter-comparison of NASA-SSE and DLR-ISIS
satellite-derived long-term data by Meyer et al. (2006, Proc. of the ASES
Solar 2006 conference, Denver July 2006) time-series shows very good
agreement for global horizontal irradiance (GHI). The mean bias on average
is 0.4 W/m², which is equal to an annual sum of less than 4 kWh/m². Earlier
reports on reduced GHI can not be confirmed by either satellite datasets.
SSE and ISIS reveal that from 1984 to 2004 there is more likely a slight
worldwide increase, but this result is not significant. At least DLR-ISIS
shows a strong increase in direct normal irradiance (DNI) for many regions
in the world. DNI derived from NASA-SSE, however, requires reprocessing,
which is now underway.
C.3: Forecasting of Solar Radiation
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A conference paper on now-casting and short-term forecasting
(Lorenz, et al., Proc. of the ASES Solar 2006 conference, Denver July 2006)
was presented.
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A PhD thesis 6/2006 (M. Girodo) and paper (7/2007) on
mesoscale modeling for short-term forecast of solar irradiance has been
completed.
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NASA/LaRC, together with SUNY/A, evaluated NOAA’s digital
forecast database for applicability for solar energy forecasts (paper
presented at Solar 2006).
Work Planned for
2007
C.1: Improving Satellite Methods for Solar Radiation Products
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Universität Oldenburg is further improving a scheme to better
account for cloud shadow effects. This strongly enhances correlation of
ground-based measurements and satellite retrieved solar irradiance in hourly
and shorter time resolution.
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DLR IPA will process a synthetic satellite image based on 1-D
and 3-D radiative transfer calculation to provide a test bed for further
development of new retrieval schemes.
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NASA’s new SSE Release 6.0 is to include results from the
GEWEX SRB SW Rel 2.7 which is scheduled for release in Dec 2006/Jan 2007.
Release 6.0 is a complete list of atmospheric inputs used to generate the
current version that will be provided to the team as necessary.
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In collaboration with SUNY/A, NASA/LaRC plans to analyze
methods to improve direct and diffuse SW irradiance estimates for clear and
cloudy sky conditions.
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Development and evaluation of the effect of improving angular
distribution models on the solar irradiance estimates for the NASA method.
Angular Distribution Models inherently include uncertainties due to lack of
homogeneity.
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Development and evaluation of new aerosol inputs to solar
irradiance (NASA/LaRC, and DLR DFD).
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Meteotest plans to generate worldwide climatologies of AOT550
and TL (2000-2005) based on MODIS, MISR and AERONET.
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Continued testing and development of a method to impose
higher resolution solar resource maps with long-term data sets
(collaboration with SUNY/A).
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JRC, EC, is implementing high-resolution digital elevation
models (SRTM-3) to study the influence of terrain on modeling the solar
radiation at various resolutions.
C.2: Climatological Analysis of Solar Resources
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Report on results of the GEWEX Radiative Flux Assessment
project relevant to the Task in terms of long-term data sets. This project
aims at benchmarking the accuracy and precision of long-term
top-of-atmosphere and surface radiation fluxes of which solar irradiance is
a key component. GEWEX-RFA will study many solar irradiance data sets
participating in the Task and will provide a framework for evaluation of
other datasets.
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Meteotest will provide Task 36 with data after 1990 from the
GEBA (Global Energy Balance Archive) dataset. These time-series should be
analyzed for changes.
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Long-term time series analysis of NASA’s solar irradiance
datasets using GEBA and other measurements (see Subtask A).
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Begin evaluation of possible use of NASA seasonal to
inter-annual forecast model for seasonal irradiance forecasts.
(Collaboration with NASA Goddard Space Flight Center (GSFC)).
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Begin evaluation of Climate scenario solar irradiance
forecasts (collaboration with NASA Goddard Institute for Space Science (GISS).
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Meteocontrol: Germany plans to better describe year-to-year
variability in solar radiation, as input for risk calculation.
C.3: Forecasting of Solar Radiation
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Report by Universität Oldenburg on now-casting up to 6 hours
based on extrapolation of cloud motion vectors derived from satellite images
(12/2006)
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Continued evaluation of short-term forecasts comparing the
USA NOAA National Digital Forecast Database parameters with measurements.
Identify other short-term models and missing parameters needed for improved
solar irradiance forecasts (collaboration with SUNY/A)
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Continued evaluation of operational global radiation
forecasts with the WRF/ MM5 model (Meteotest).
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Build up of a model for WRF model global radiation output
statistics (Meteotest).
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